1. Field of the Invention
The present invention relates to a double structure and material of artificial molars designed such that at the point of time when artificial teeth for molars formed of synthetic resins, ceramics or composite materials thereof assumes a occlusal geometry accommodative to various oral functions in the oral cavity of a patient, only the occlusal regions of the artificial teeth are replaced by metals, composite resins, castable ceramics or amalgams of durability with respect to abrasion, and such replacement of the occlusal regions.
2. Description of the Prior Art
Occlusion expressed in terms of the dentition relation of the upper and lower jaws has an appreciable influence upon mastication, face, pronunciation, etc. In particular, the masticatory function of importance to the lengthening of the life span of humans is one of the lower jaw's motions whose upper limit is defined by masticatory relations. In other words, in order to have the masticatory function improved, it is required to establish the occlusal relation in harmony with jaw motions inherent in patients, restricted by the temporomadibular joints and ligaments and reflectively controlled by the mechanisms of nervous and muscular regulation. If that occlusal relation is in disharmony, then fracturing of dents or periodontal tissues or disorders of the temporomadibular joints or muscle will be caused.
In the preparation of protheses, much importance is also attached to the occlusal geometries thereof in harmony with the masticatory motions of patients, like natural teeth. The same is said of plate dentures. That is, the occlusal geometries of artificial teeth applied to the plate dentures directly govern the functions of the completed dentures. Included in artificial teeth are porcelain teeth, resin teeth, metal teeth and so on. Porcelain teeth, which excel in wear resistance, are frequently used in the form of complete dentures, but need an extended period for occlusal equilibration and give rise to not a few failures of the completed denture artificial teeth. It is difficult to use porcelain teeth for partial dentures, since there is an increase in the amount of artificial teeth to be cut off during alignment. As a result, resin teeth are generally used for plate dentures, esp., partial dentures.
In the actual making of dentures, artificial teeth are aligned or milled-in on an articulator serving as a simulator of a patient prepared on the basis of the records of bite taking or jaw motions, while taking into consideration the health conditions of musculi masticatorii, the temporomandibular joints, the alveolar mucosae and so on, the defects of dents, occlusion, root implantation and inclination of residual teeth, and the like. If such a series of operations are properly carried out, the function of the completed plate denture will then be well-balanced in the oral cavity. However, as the resin denture is used in the oral cavity, its occlusal surface is attrited away, so that its initial geometry is fractured within a relatively short period, resulting in an extreme lowering of the denture's masticatory function. In order to cope with this, it has been carried out to replace the occlusal surface by a metal before attrition proceeds to an extreme level. Specifically, the denture is again attached to an articulator with a model, and each artificial tooth is suitably cut off on its occlusal surface, followed by placement of wax. The geometry of wax is patterned or determined by functional milling-in with an antagonistic dentition, and the wax pattern is fitted to a metal casting. This is called a metal occlusal table. However, not until now is any method for preparing occlusal tables with castable ceramics, composite resins or amalgams by such a technique available.
The following problems arise in connection with the method for making metal occlusal tables wherein the occlusal regions of artificial teeth are suitably cut off to receive softened wax, against which a core or antagonist is pressed to obtain functional occlusion.
(1) The optimum functional occlusion obtainable from natural mastication (hereinafter called normal mastication) through food in normal life is not achieved on articulators using softened wax or by pressing techniques in clinics, even though it is possible to obtain a metal occlusal table formed of a durable metal. Hence, serious problems such as "impossible to bite" and "lingering pain" arise.
This is caused for the following three reasons.
(1--1) In order to obtain the optimum functional occlusion, it is required that patients continue normal mastication over one week to one month. However, pressing techniques using softened wax make it impossible for patients to continue normal mastication over one week to one month in view of the thermal, mechanical and chemical properties of the material of the wax, because the wax assumes plasticity at temperatures from the bodily temperature of humans to warm water and cannot stand up to normal mastication in terms of strength.
(1-2) Regardless of how much patients cooperate with dentists, recording of central occlusion is only achieved in a tense atmosphere prevailing in clinics, no matter how occlusal equilibration or tapping with wax is repeated. It is thus nearly impossible to obtain the optimum functional occlusion of normal mastication in a relaxed atmosphere.
(1-3) Manual or automatic milling-in of regions marked by articulating paper as unfit contact with articulators only gives face-to-face contact occlusion without food. This implies that normal mastication cannot actually and precisely be reproduced with the articulators in spite of the fact that they are orginally a simulator of the oral jaws and craniums of humans.
(2) It is impossible for any person to make the metal occlusal tables for a short time with facility, since considerable skill is needed for the method for making the metal occlusal tables wherein the occlusal regions of artificial teeth are suitably cut out to receive softened wax, against which a core or antagonist is pressed to obtain functional occlusion.
(3) According to the method for making the metal occlusal tables wherein the occlusal regions of artificial teeth are suitably cut out to receive softened wax, against which a core or antagonist is pressed to obtain functional occlusion, they are likely to deform at the time of attachment or detachment without a thickened layer of softened wax, because of its low functional strength. This may lead to an increase in the amount of the metal used and, hence, a rise in the cost of the material.
(4) An increase in the amount of the occlusal surfaces of artificial teeth to be cut out attracts much attention to a metallic color, as viewed in the buccal direction, and poses an aesthetic problem in connection with premolar teeth in particular.